Thursday, March 22, 2012

The Greenland ice
sheet is likely to be more vulnerable to global warming than previously
thought. The temperature threshold for melting the ice sheet completely is in
the range of 0.8 to 3.2 degrees Celsius of global warming, with a best estimate
of 1.6 degrees above pre-industrial levels, shows a new study by scientists
from the Potsdam Institute for Climate Impact Research (PIK) and the
Universidad Complutense de Madrid. Today, already 0.8 degrees of global warming
has been observed. Substantial melting of land ice could contribute to
long-term sea-level rise of several meters and therefore it potentially affects
the lives of many millions of people.

The time it takes
before most of the ice in Greenland is lost strongly depends on the level of
warming. "The more we exceed the threshold, the faster it melts,"
says Alexander Robinson, lead-author of the study now published in Nature Climate
Change. In a business-as-usual scenario of greenhouse-gas emissions, in the
long run humanity might be aiming at 8 degrees Celsius of global warming. This
would result in one fifth of the ice sheet melting within 500 years and a
complete loss in 2000 years, according to the study. "This is not what one
would call a rapid collapse," says Robinson. "However, compared to
what has happened in our planet's history, it is fast. And we might already be
approaching the critical threshold."

In contrast, if
global warming would be limited to 2 degrees Celsius, complete melting would
happen on a timescale of 50.000 years. Still, even within this temperature
range often considered a global guardrail, the Greenland ice sheet is not
secure. Previous research suggested a threshold in global temperature increase
for melting the Greenland ice sheet of a best estimate of 3.1 degrees, with a
range of 1.9 to 5.1 degrees. The new study's best estimate indicates about half
as much.

"Our study
shows that under certain conditions the melting of the Greenland ice sheet
becomes irreversible. This supports the notion that the ice sheet is a tipping
element in the Earth system," says team-leader Andrey Ganopolski of PIK.
"If the global temperature significantly overshoots the threshold for a
long time, the ice will continue melting and not regrow -- even if the climate
would, after many thousand years, return to its preindustrial state." This
is related to feedbacks between the climate and the ice sheet: The ice sheet is
over 3000 meters thick and thus elevated into cooler altitudes. When it melts
its surface comes down to lower altitudes with higher temperatures, which
accelerates the melting. Also, the ice reflects a large part of solar radiation
back into space. When the area covered by ice decreases, more radiation is
absorbed and this adds to regional warming.

The
scientists achieved their insights by using a novel computer simulation of the
Greenland ice sheet and the regional climate. This model performs calculations
of these physical systems including the most important processes, for instance
climate feedbacks associated with changes in snowfall and melt under global
warming. The simulation proved able to correctly calculate both the observed
ice-sheet of today and its evolution over previous glacial cycles, thus
increasing the confidence that it can properly assess the future. All this
makes the new estimate of Greenland temperature threshold more reliable than
previous ones.